1. Field of the Invention
The present invention relates to a method and apparatus for dicing of thin and ultra thin semiconductor wafers using ultrafast pulse laser, and more specifically it relates to an apparatus and method for dicing using ultrafast pulse laser directly from an oscillator without an amplifier, operating in picosecond and femtosecond pulse width modes.
2. Description of the Related Art
Dicing of Thin and Ultra Thin Silicon Wafer
The semiconductor industry is moving towards thin silicon in various fields. The bulk of the silicon plays no role in the performance of the circuit, and hence the semiconductor industry is moving toward thin silicon. The reasons include a desire to increase the density of the integrated circuit by stacking the circuit; to conduct heat away from the active area by moving the device closer to the metal heat sink by reducing the thickness of silicon; and to use smart cards and other applications that need thin silicon wafers.
Traditionally a saw blade such as a diamond saw is used for silicon wafer dicing. But as the thickness of the wafer decreases below 100 micrometer, saw dicing leads to chipping and breaking of the dies. Also the dicing speed reduces as thickness of the silicon wafer reduces in the case of saw dicing. Alternatively the silicon is diced by saw and ground to the desired thickness, but this process leads to severe backside chipping during the grinding process.
Concerns about ultra-thin IC packaging dicing include wafer breakage. Even a small force will cause breakage, the worst case in semiconductor manufacturing. Below the thickness of 50 μm contact dicing method is not feasible due to wafer breakage. Water is extreme harmful, and round corners are required to enhance the die strength
Saw Blade Dicing
Traditionally a saw blade such as a diamond saw is used for silicon wafer dicing as disclosed in U.S. Pat. Nos. 6,277,001, 6,361,404, 6,676,491, 6,500,047, 6,465,158, and 6,528,864. But as the thickness of the wafer decreases below 100 micrometer, saw dicing leads to chipping and breaking of the dies. Also the dicing speed reduces as thickness of the silicon wafer reduces in case of dicing saw. Alternatively the silicon is diced by saw and grinded to the desired thickness, but this process leads to severe backside chipping during grinding process.
Nanosecond Laser (Nd—Yag) Dicing
In order to overcome the problem associated with a saw for thin silicon dicing, a diode pumped solid state Nd—Yag laser of nanosecond pulse width has been used (U.S. Pat. No. 6,676,878). A long pulse laser increases the dicing speed and reduces the dicing kerf width, but it has limitations due to long pulse width (nanosecond pulse width).
The advantages of this technique include a reduced kerf width, typical kerf=25 μm, a high speed at thin wafer thickness, and round corners are possible.
The problems of this technique include chipping and micro cracks. Moreover, the heat affected zone alters the property of the materials in the vicinity of the machined surface resulting in reduced material strength and hence the die strength. There is also damage to the surrounding circuit because of heat diffusion. There is debris due to spattered molten material. There is water usage which is unavoidable since post dicing washing is needed. There are recast deposits on the sidewall, which is vital for semiconductor application. This is also low die strength due to heat diffusion
Dicing of Low-K Dielectric Semiconductor Wafer
The conventional silicon dioxide dielectrics are inadequate for the future needs. Better performing ICs mandates the introduction of several new materials into the device structure, including interlayer dielectrics with low dielectric constant. These low-K dielectrics are insulating materials that are specially designed to reduce the capacitance between the copper lines on the chip. An insulator with a low dielectric constant, than the value of 4.2-4.5 for silicon dioxide is required primarily for the realization of the full benefit of the copper dual-damascene technology. The dielectric constant value of the insulating material decreases with the reduction in the node width. The fragility and poor adhesion of low-K dielectric cause serious difficulty in dicing of these layers, restraining manufacturers from introducing low-K dielectrics into their product lines. Also advanced package such as wafer scale or wafer level packaging which consist of a thick polymer layer on top of silicon substrate. When dicing saw is used for dicing wafers with low-K dielectric, large tensile and shear stress are imparted at the cut zone which leads to significant cracking and adhesion loss leading to delamination and chipping of the metal and low-k layers. In order to eliminate this problem diode pump solid state laser of wavelength ranging from 1100 to 250 nm of 1-100 nanosecond pulse width was used for pre scribing the low-k dielectric and metal layer before dicing using a saw blade. Due to long laser pulse width and low absorbability of the low-k dielectric layer, laser scribing using long pulse width laser (more than 100 pico second) leads to delamination and chipping. Although the delaminating and chipping reduces with the reduction in the wavelength of the long pulse laser (more than 100 ps), but these problem cannot be completely eliminated over the entire dicing lane or wafer. As the nod size reduces and hence the low-k dielectric constant value reduces, it is easy to chip and delaminate when long pulse laser is used for scribing.
Some of the drawbacks, associated with the dicing of low-K dielectric semiconductor using nanosecond laser and saw, include two steps (laser scribing followed by saw dicing), and positioning error occurs when aligning saw to laser scribed lane. Other drawbacks include chipping, delamination and debris.
Ultrafast Laser Processing
Amplified short pulse laser of pulse width 100 picosecond to 10 femtosecond are being used in general applications to overcome the problem of long pulse laser. One a advantage of short pulse lasers in comparison to long pulse laser includes the energy does not have the time to diffuse away and hence there is minimal or no heat affected zone and micro cracks, since the duration of short pulse laser is shorter than the heat dissipation time. There is also negligible thermal conduction beyond the ablated region resulting in negligible stress or shock to surrounding material. Since there is minimal or no melt phase in short pulse laser processing, there is no splattering of material onto the surrounding surface. There is no damage caused to the adjacent structure since no heat is transferred to the surrounding material. There are no undesirable changes in electrical or physical characteristic of the material surrounding the target material. There is no recast layer present along the laser cut side walls, which is vital for semiconductor application. Ultrafast laser processing eliminates the need for any ancillary techniques to remove the recast material within the kerf or on the surface. The surface debris present does not bond with the substrate and are easy to remove with conventional washing techniques. Machined feature size can be significantly smaller than the focused laser spot size of the laser beam and hence the feature size is not limited by the laser wavelength.
Short pulse laser can be broadly divided in to two categories. The first is femtosecond pulse width laser (ranging from 10 fs-1 ps), and the second is pico second pulse width laser (ranging from 1 ps-100 ps).
The femtosecond laser system (which is generally a Ti-sapphire laser) generally consist of a mode locked femtosecond oscillator module, which generates and delivers femtosecond laser pulse of in the order of nanojoule pulse energy and 10-200 MHz repletion rate. The low energy pulse is stretched in time prior to amplification. Generally the pulse is stretched to Pico second pulse width in a pulse stretcher module, using a dispersive optical device such as a grating. The resultant stretched beam is then amplified by several orders of magnitude in the amplifier module, which is commonly called as regenerative amplifier or optical parameter amplifier (OPA). The pump lasers generally used to pump the gain medium in the amplifier are Q-switched Neodymium—yttrium—lithium—floride (Nd—YLF) laser or Nd: YAG laser with the help of diode pump laser or flash lamp type pumping. The repletion rate of the system is determined by the repletion rate of the pump laser. Alternatively if continuous pumping is used, then the repetition rate of the system is determined by the optical switching within the regenerative amplifier. The resultant amplified laser pulse is of Ps pulse width is compressed to femtosecond pulse width in a compressor module. By this means femtosecond pulse of mille joules to micro joules of pulse energy of repletion rate 300 KHz to 500 Hz and average power less than 5W are produced.
The amplified femtosecond pulse has been used widely for micro machining applications such as U.S. Pat. Nos. 6,720,519, 6,621,040, 6,727,458 and 6,677,552. However, it suffers from several limitations, which prevents it from being employed in high volume manufacturing industrial applications. The system is very unstable in terms of laser power and laser pointing stability. Laser stability is very essential in obtaining uniform machining quality (Ablated feature size) over the entire scan field. The average laser power is too low to meet the industrial throughput. The Amplified femtosecond laser technology is very expensive, which will increase the manufacturing cost considerably. The down time of the system is high to the complexity of the laser system. There is a large floor space required for the laser system. There is relatively poor feature size and depth controllability due to laser power fluctuation. Experienced and trained professionals are required for the maintenance of the system.
In contrast, an amplified pico second laser system comprised of a pico second oscillator, which delivers picosecond laser of nanojoules pulse energy, is amplified by a amplifier. The pump lasers generally used to pump the gain medium in the amplifier are Q-switched Neodynium—yttrium—lithium—floride (Nd—YLF) laser or Nd: YAG laser with the help of diode pump laser or flash lamp type pumping. The repletion rate of the system is determined by the repletion rate of the pump laser. Alternatively, if continuous pumping is used then the repetition rate of the system is determined by the optical switching within the regenerative amplifier. The resultant amplified pulse has a repletion rate ranging from 500 Hz to 300 KHz of average power 1 to 10 W.
An amplified picosecond laser is simple and compact in comparison to an amplified femtosecond laser, but it has limitations. The amplified picosecond laser is also more stable than an amplified femtosecond laser system, and it is still unstable in terms of laser power and laser pointing stability to meet the needs for industrial high volume manufacturing applications. Laser stability is very essential in obtaining uniform machining quality (ablated feature size) over the entire scan field. The amplified picosecond femtosecond laser technology also is cheaper than amplified femtosecond laser system, but it is still expensive, which will increase the manufacturing cost considerably. There is relatively poor feature size and depth controllability due to laser power fluctuation. The down time of the system is high. Large floor space for the laser system is needed. Experienced and trained professionals are required for the maintenance of the system
Femtosecond laser with very low fluency is a promising machining tool for direct ablating of sub-micron structures. Fundamental pulses emitting from oscillator can be used to create nano-features. But due to short time gap between the successive pulses, there is a considerable degrade in the machining quality, which may be explained as below.
At the end of the irradiation of an individual laser pulse, surface temperature rises to Tmax. Due to thermal diffusion, the surface temperature decays slowly and eventually reduces to the environment temperature T0. The time span of the thermal diffusion τdiffusion can be determined by the one-dimensional homogeneous thermal diffusion equation. In the case of multi-shot ablation, if the successive pulse arrives before τdiffusion (t<τdiffusion), the uncompleted heat dissipation will enhance the environment temperature. The environment temperature after n laser shots for a pulse separation of t at a time just before the next (or (n+1)th) shot can be expressed by T0(n)=T0+nδT, where, δT is the temperature rise due to un-dissipated heat at the end of a pulse temporal separation.
The actual surface temperature Tmax (n) after n successive pulses can be written asTmax(n)=T0(n)+Tmax 
The enhanced surface temperature of the ablation front will cause over heating and deteriorate the quality of ablation. In the case of via drilling application, such over heating deteriorate the geometry of via, causing barrel at the bottom of the hole.
The longer the time between successive pulses, the less is the effect of the thermal coupling enhancing the surface temperature. When pulse separation t is long enough that the heat diffusion outranges the thermal coupling, the machining quality of multi-shot ablation will be as good as that of single-shot ablation.
In fact, thermal coupling effect of multi-shot ablation was observed not only for nano-second pulses but also for ultrafast laser pulses. Fuerbach, reported that to avoid degrading of machine precision due to heat accumulating 1 μs pulse separation should be given for femtosecond pulses ablation of glass.